An MRI device is a medical diagnostic imaging apparatus, which induces magnetic resonance in nuclei in an arbitrary section of a test subject, and provides a tomogram for the section from generated magnetic resonance signals. When a high-frequency magnetic field is irradiated on a living body rested in a static magnetic field by a high-frequency coil (RF coil) with applying a gradient magnetic field, atomic nuclei in the living body, for example, hydrogen nuclei, are excited, and when the excited hydrogen nuclei return to the equilibrated state, circularly polarized magnetic fields are generated as magnetic resonance signals. These signals are detected with an RF coil and subjected to signal processing to visualize hydrogen nucleus distribution in the living body. In order to obtain an image of high precision and high resolution, improvements in receiving sensitivity, Signal-to-Noise (SN) ratio, etc. are desired for the RF coil for detecting magnetic resonance signals.
In recent years, a technique called parallel imaging is used, in which a plurality of coils showing different sensitivity profiles are used to execute an imaging sequence with reduced phase encoding steps, and aliasing artifacts of images are eliminated by an operation utilizing the sensitivity profiles of the RF coils (for example, refer to Non-patent document 1). According to the parallel imaging, imaging time is shortened to the extent that the phase encoding steps are reduced. Also for this parallel imaging, improvements in receiving sensitivity and SN ratio of the RF coil are desired.
Moreover, there are imaging techniques aiming at enlargement of detection range among those utilizing a plurality of RF coils. For example, there is a technique of using a plurality of surface coils to detect magnetic resonance signals from a test subject, and synthesizing the output signals of the surface coils to obtain an image with high SN ratio for a wide range (for example, refer to Patent document 1). In this technique, a plurality of surface coils are disposed near a region of the object of the imaging, and magnetic resonance signals from the test subject are detected by using a plurality of those surface coils. Magnetic resonance signals detected with each surface coil are subjected to a signal processing to generate image data for every surface coil. Then, pixel data corresponding to the same position in the generated images are multiplied with weighting coefficients obtained from the sensitivity profiles of the surface coils, and added.
When imaging is performed by using a plurality of surface coils, if electromagnetic coupling arises between a plurality of the surface coils to be used, high frequency signals generated in one surface coil are also induced in the other surface coil at a certain ratio. Noises are increased by the induced high frequency signals, and the SN ratio of output signals is reduced. Therefore, it is necessary to prevent electromagnetic coupling between surface coils.
As techniques for preventing electromagnetic coupling between coils, there are a method of disposing adjacent coils with partial overlap and controlling areas of the overlap of the loops (for example, refer to Non-patent document 2), a method of serially inserting inductors into a loop conducting wire of each coil at close positions to cancel the electromagnetic coupling (refer to, for example, Patent document 2), a method of inserting capacitors into loop conducting wires of adjacent coils so that a part of the capacitors is shared by the coils and tuning values of the capacitors to prevent the electromagnetic coupling (refer to, for example, Non-patent document 3), a method of connecting parts of adjacent coils with a neutralization circuit comprising capacitors (refer to, for example, Patent document 3), a method of disposing a figure-of-eight coil or a loop coil so as to overlap with two adjacent coils (refer to, for example, Patent document 4), a method of using a low input impedance of an preamplifier for amplifying magnetic resonance signals detected by coils (refer to, for example, Non-patent document 2), a method of connecting an impedance controlling circuit between a coil and a preamplifier (refer to, for example, Non-patent document 4), and so forth.
When loops of a plurality of RF coils used for the detection partially overlap with each other, the sensitivity profiles of the overlapping portions of the RF coils shows the same tendency, and difference of the sensitivity profiles of both RF coils becomes small. In the parallel imaging, if the difference of sensitivity profiles becomes small, the SN ratio is degraded in the operation for eliminating the aliasing artifacts of images. Therefore, it is known that, in the parallel imaging, disposition of a plurality of RF coils with intervals provides a higher SN ratio in an image to be obtained compared with disposition of them with partial overlap (refer to, for example, Non-patent document 5).